In my last blog, A Quick Overview of Radiation Effects, I introduced concepts around radiation effects in semiconductors. The focus in that last blog was on cumulative effects which tend to be more permanent in nature. In this blog we will look more at single even effects (SEE) that are generally more temporary in nature. While some of these single event effects can cause permanent damage the focus of this blog series will be more on the non-permanent single event effects. These effects generally last short periods of time upon which afterward the device can return to normal operation. We will discuss some SEEs that can have permanent effects just for the sake of completeness.
Recall that when a device is placed into the harsh environment of space it can expect to see radiation that can cause different types of undesired behavior. These effects need to be understood before a device is placed into an application in space. For this reason testing is performed at radiation facilities such as the Cyclotron at Texas A&M University. There is also a facility at the University of California at Berkeley. These facilities at UC-Berkeley and Texas A&M are generally used for observing single event effects.
Let’s now take a look at the various single event effects. When testing for these types of upsets the device is exposed to certain levels of radiation expressed in LET (Linear Energy Transfer) in MeV-cm2 /mg. Generally, the LET applied ranges from a few MeV-cm2 /mg up to 80 MeV-cm2 /mg and sometimes above. The onset of SEEs are found by beginning with a lower LET value and increasing the level while monitoring for the SEEs. The LET is increased to reach a saturation point for the SEEs. From this data a Weibull fit curve can be generated to see a visual plot of the SEE behavior of the device. This plot shows where the device begins to exhibit SEEs, how the SEEs increase with the increase in LET, and the saturation point at which the number of SEEs do not increase with the increase in LET. Just as with cumulative effect testing the device would be biased into normal operating mode to emulate the same conditions the device would see in its application in space. The single event effects observed during this testing include single event latchup (SEL), single event upset (SEU), single event failure interrupt (SEFI), single event transient (SET), single event burnout (SEB), and single event gate rupture (SEGR).
Single Event Effects Due to Radiation Exposure
Single event latchup (SEL) is pretty much as it sounds, the device exhibits a latchup condition where there is an abnormally high supply current. Generally this occurs much like it does when a device typically latches up only in this case the ions from the radiation are what cause the latchup. One such example of an SEL would be when an ion strike induces a parasitic BJT (bipolar junction transistor) that causes a high current to flow from power to ground. This type of SEE can result in permanent damage to the device, however measures can be employed to prevent the SEL from causing permanent damage. If no permanent damage occurs form the SEL then a power cycling of the device can restore the device to normal operation.
Single event upset (SEU) is a soft error that occurs during radiation exposure where there is a quick recovery by the device. This could be something such as a bit upset or multiple bit upsets in a configuration register. This is non-permanent and short term in duration. These upsets do not require a device reset to restore normal operation.
Single event failure interrupt (SEFI) is also a soft error but in this case the device can reset clearing device settings, malfunction in some detectable way, or completely lock up. In the case of a SEFI the device may require a reset to restore normal operation but does not require a power cycle.
Single event transient (SET) is a brief voltage spike at a node in a circuit caused by an ion strike. In this case it could be something such as the output of a driver or amplifier having a momentary increase in output voltage. This type of event is temporary in nature and the device returns to normal operation after a short time without requiring a device reset or power cycle. The transient can work its way through the device and affect the next device in the signal chain.
Single event burnout (SEB) is when an ion strike causes a high current to be generated in a small area of an integrated circuit. The high current results in damage to the device and can be a catastrophic failure. Generally, these types of SEEs are seen with power devices.
Single event gate rupture (SEGR) is when an ion strike causes a degradation in the gate oxide of a MOSFET resulting in current flow through the gate of the transistor. The event can cause break down of the oxide resulting in increased leakage current. If severe enough the MOSFET could be damaged enough that the device no longer operates properly and is catastrophically damaged. These events typically occur power MOSFET devices.
This is by no means an extensive look at the details of the various SEEs that result from radiation exposure in integrated circuits. There are many journal papers and technical articles offer much more detail on these SEEs. For the purpose of this blog, the discussion is kept at a high level to simply give an overview of what SEEs are observed. Over the course of the upcoming blogs we will look mainly at SEL, SEUs, SEFIs, and SETs as they apply to the operation of an ADC.
As we continue the discussion we will look at how these particular SEEs apply to ADCs and how they are observed. Given my background in ADCs it makes a nice logical connection to put these two areas together. I think some real examples will also help illustrate radiation effects that occur with integrated circuits.
If you would like to learn more on single event effects I would encourage you to visit the JEDEC page and search for the particular SEE you are interested in. You can also visit NASA’s Radiation Effects & Analysis page that has more information as well as several links regarding SEEs.